At the beginning of the movie, in Germania, Maximus is seen riding a horse with STIRRUPS. Also later on in the film when the soldiers ride into Maximus' home they too are riding with stirrups. The Romans DID NOT have stirrups!! I do A-Level Archaeology and I have done a project on the Roman cavalry so I know what I'm talking about! Stirrups didn't appear until much later towards the end of the Roman Era when the Mongols developed stirrups.

Yes, this is an anachronism, but let's give these people a break. It's hard to ride a horse without stirrups, especially if you haven't been trained that way and you have to do border-line stunt work such as jumping the horse over fire, etc. They used stirrups because it made the filming easier, and only people like you are going to notice or care that it's inaccurate.

I ride horses on a show circuit, doing jumping, and know that riding without stirrups, for anyone, is difficult. Russel Crowe, in the scenes in which he is riding, does a posting trot (posting is standing up and forward slightly in the saddle and sitting back down to the beat of the horse's trot). He rides during conversations with other characters, so using a body double would have been impractical. The idea of an actor to be able to ride the horse in good form, post, gallop, and especially jump, without stirrups, is absurd. So the choice was in the director's hands: either be criticized for using a body double too often, or for using stirrups.